Some key ideas in the kinetic theory of gasses arise from making some assumptions about how molecules move about in the gas. This demonstration uses plastic beads in the MMD to illustrate some of these assumptions.


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The kinetic theory of gases treats a gas in a container as a large number of very small particles moving around within the container. Beads in the MMD apparatus move around in a similar fashion, and can be used to illustrate how molecules in a gas behave. Like the beads, 1) molecules are generally separated from one another by a large distance, 2) some molecules move fast and some move more slowly, 3) molecules travel in a straight line until they collide with other molecules or with the container, and 4) the molecules travel in random directions.

Point number one leads to a consequence for the compressibility of a gas. Unlike the molecules in a solid or in a liquid, the molecules in a gas generally have a lot of empty space between them. Gas molecules can be "packed" into this space, so gasses are compressible.


Because a gas is a large number of very small particles moving very quickly and in random directions, the gas will tend to spread out to all the space available to it. This means a gas will take the shape of its container.

The pressure (P) of a gas is an important idea in physics, and is a consequence of points two, three, and four. Point two says that molecules collide with the walls of their container. These collisions each exert a force on the wall of the container. Points three and four say that the velocities (speeds and directions) of the molecules are randomized. In a macroscopic sample of gas containing many, many molecules, this randomness ensures that the forces on the container's walls are distributed evenly. The average total force on the container's walls (F), divided by the area (A) of the container is the pressure of the gas... P = F/A

Figure 1: Molecular Motion Demonstrator





  • Molecular Motion Demonstrator
  • 12 large blue beads



  • Set up the Molecular Motion Demonstrator on a flat surface, or overhead projector.
  • Place twelve large blue beads inside the frame. Adjust the motor speed until the "molecules" are moving fast enough to best demonstrate gas molecules moving.